An industrial metal detection system is used to detect and reject unwanted metal contamination. When properly installed and operated, it will help reducing metal contamination and improving food safety. Most modern metal detectors utilise a search head comprising a “balanced coil system”. Detectors of this design are capable of detecting all metal contaminant types including ferrous, nonferrous and stainless steels in a large variety of products such as fresh and frozen products.
A metal detection system that operates according to the “balanced coil”-principle typically comprises three coils that are wound onto a non-metallic frame, each exactly parallel with the other. The transmitter coil located in the center is energised with a high frequency electric current that generates a magnetic field. The two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed with the same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the receiver coils are connected in series with the second receiver coil having an inversed sense of winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity are cancelling out one another in the event that the system, in the absence of metallic contaminations, is in balance.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils the voltage induced in each receiver coil is changed (by nano-volts). This change in balance results in a signal at the output of the receiver coils that can be processed, amplified and subsequently be used to detect the presence of metal contamination.
The signal processing channels split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal contaminant “product effects” need to be removed or reduced. Knowing the phase of the product the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
In order to obtain information about the sort and volume of the contaminants and in order to at least partially eliminate unwanted signals caused by “product effects” or disturbances such as vibrations, it is important that the system processes the measured signals accurate signal amplitude and signal phase.
In the event that system deficiencies occur that degrade the amplitude or the phase of the processed signals, then the measurement results, which reflect the quality of the production process, are no longer reliable. Either the system may not raise an alarm if a contamination is present (false negative). Alternatively the system may raise an alarm if a contamination is not present (false positive). Hence, advanced metal detection systems are provided with equipment that allows monitoring the operation of the metal detection system.
A method for monitoring the operation of a metal detection system is disclosed in EP2439560B1. According to this method a carrier signal with the transmitter frequency and a monitoring signal with a monitoring frequency are provided to a modulation unit that suppresses the carrier signal and that provides a modulated monitoring signal, which is applied to a monitoring coil that is inductively coupled with one of the receiver coils, whose output signals are demodulated in a demodulation unit that provides the demodulated monitoring signal, which is compared in phase and/or in amplitude with a reference. In the event that a deviation between the demodulated monitoring signal and the reference exceeds a given threshold value, then an alarm signal is provided.
In WO2006/021045A1 it is explained that simultaneous operation on two frequencies enables a metal detection system to achieve higher performance both in target discrimination and rejection of false signals caused by the environment. Further, it is outlined that difficulties in the construction of multiple frequency metal detectors have prevented their proliferation, since for each extra frequency added to a conventional metal detector, a number of processing units would have to be added, thus increasing both the cost and the complexity of the detectors.
Furthermore, in view of the disclosure in EP2439560B1, such a multiple frequency metal detection system should also be equipped with a monitoring system. However, as described in WO200621045A1, adding further complexity to the metal detection system would not be desirable.